Optical corner cubes have long been used because they exhibit an alignment insensitive retroreflection of an optical beam. In the past, these corner cubes have been constructed using three flat mirrors which are positioned mutually orthogonal to each other. This mirror alignment exhibits an apex which is the point where the three mirrors come together. A beam of light which strikes the corner cube displaced from the apex will undergo three reflections and then return parallel to the incident direction except that the return beam is displaced symmetrically on the other side of the apex of the corner cube from the instant beam. Corner cubes have been used in laser systems to provide an alignment insensitive optical element. However, these corner cubes suffer from a drawback since, in many laser systems, it is desirable that the reflected beam receives a predetermined amount of optical power (usually convergence) from the reflecting optics to offset the effect of diffraction. Therefore, while the corner cube exhibits the desirable property of being a retroreflector with great alignment insensitivity, it has the undesirable property of being analogous to an optical flat inasmuch as a divergent optical beam striking the corner cube will continue to diverge as it leaves the corner cube.
In laser systems, for example, spherical mirrors are often used to impart the desired convergence. However, it is well known that spherical mirrors produce low distortion convergence only when used close to the optical axis of the sphere. When spherical mirrors are used even slightly off axis, astigmatisms are introduced which result in distortion of the optical beam. These astigmatisms increase in magnitude as a spherical reflector is used further off axis. This problem of spherical mirrors has largely limited their use to near normal incidence optical systems.
Therefore, it would be desirable to have an optical device which had the retroreflecting properties of a corner cube and yet, the focusing properties of a spherical mirror used on axis.